All human learners with normal cognitive capacities who are exposed to language input in childhood manage to acquire a language that looks very much like their input. However, at present we have little idea how, exactly, learners accomplish this feat. We have little understanding of the nature of the learning mechanisms that underlie and support language learning. This is the focus of the research in this proposal. Three general questions guide the research.(1) What are the constraints on language learning mechanisms? (2) Are the learning mechanisms involved in language acquisition specific to language or are they of a more general nature? (3) Do these mechanisms change over time as learners age, and if 'so, how? The proposed research examines these questions by investigating the learning of probabilistic and inconsistent patterns in the input using miniature artificial languages. This provides a rather unique view of the constraints on learning mechanisms, examining the limits of what can and cannot be learned. Previous work has shown that learners can acquire probabilistic patterns, however they sometimes impose consistency on variation. Moreover, children are more likely to change such patterns than are adults. The present research expands on the earlier results, asking about the nature of the interaction between learners and input. Series 1 asks about the nature of the input that makes some inconstancy learnable and some not. The studies examine the limits of veridical learning of inconsistent patterns in languages, asking questions about the specificity of computations learners can perform, the representations over which such computations can be performed, and the effect of prior domain-specific knowledge. Series 2 asks about the nature of the learner, asking why learners regularize over inconsistency at all.In particular, the studies examine whether working memory constraints, rather than constraints stemming directly from the learning mechanisms themselves, are an important factor influencing whether learners acquire the variation or instead impose regularity. Both series will be conducted with adults and children to examine how learning changes over development. Although the input in the proposed studies is somewhat atypical, the results from these studies will contribute to our understanding of the learning mechanisms involved in language acquisition more generally, and ultimately, this increases our understanding of both normal and disordered acquisition.